To develop cotton germplasm with improved yield under drought conditions is one of the major goals for cotton breeders. The main purpose of this study was to evaluate 96 cotton genotypes for drought tolerance by measuring yield performance under deficit (water-limited) and full irrigation conditions. The field experiment was conducted under full (FI-100) and deficit (DI-50) irrigation conditions during the two growing seasons of 2011 and 2012 at the Agricultural Research Station of Adnan Menderes University, Aydin, Turkey. The mean data on performance of 96 different cotton genotypes showed the existance of considerable genotypic variations for yield, yield components, and drought tolerance indices. Correlation and regression analysis indicated that cotton genotypes characterizied with high GMP and low DSI could be selected as a potentially droght-tolerant genotypes. It is concluded from the present studies that, based on biplot analysis, 20 genotypes were found highly susceptile to water stress, 26 genotypes were highly susceptible to water stress but produced high yield in full irrigation, and 23 genotypes were not only water stress tolerant but also give maximum seed cotton yield. However, GC 555, Nieves, DAK-66/3, MS-30/1, Nazilli M-503, Zeta 2, Eva, NIAB 999, and Delta Diomand were found highly water stress tolerant because of maximum GMP and minimum DSI values. These genotypes could be exploited as genetic resources in breeding programs aiming to improve drought tolerance in cotton.

Cotton is an economically important commodity for nearly fifty industries including the textile sector which is largely based on cotton fiber. Identification of markers linked to loci for fiber traits under drought stress may be particularly beneficial because such loci could provide the genetic adaptability needed to produce good fiber under water limitation. In the present study, 177 simple sequence repeat (SSR) markers were used to detect significant quantitative trait loci (QTLs) linked to 11 fiber quality and plant structure traits in a panel of 99 Upland cotton (Gossypium hirsutum L.) genotypes using GLM and MLM analysis. The fiber quality traits, including fiber length (FL), fiber fineness (FF), fiber strength (FS), fiber elasticity (FE), fiber uniformity (FU), spinning conversion index (SCI), earliness (EAR), 1st position boll retention (1st PBR), 2nd position boll retention (2nd PBR), total boll number (TBN) and plant height (PH), were tested under both well-watered and water-limited irrigations in two locations. At both locations, GLM identified a total of 74 and 70 QTLs under well-watered and water limited conditions, respectively, at p ≤ 0.005. MLM detected seven and 23 QTLs under well-watered and water-limited conditions, respectively. Of the identified QTLs, some QTLs were detected in both locations: three for well-watered and two for water-stress conditions. Moreover, a total of 19 QTLs were stable under both watering-regimes. The QTLs identified herein could be useful in the development of cotton cultivars that have adaptability to drought conditions worldwide.

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Drought causes serious yield losses in cotton production throughout the world. Association mapping allows identification and localization of the genes controlling drought-related traits which will be helpful in cotton breeding. In the present study, genetic diversity analysis and association mapping of yield and drought traits were performed on a panel of 99 upland cotton genotypes using 177 SSR (simple sequence repeat) markers. Yield parameters and drought tolerancerelated traits were evaluated for two seasons under two watering regimes: water-stressed and well-watered. The traits included seed cotton yield (SCY), lint yield (LY), lint percentage (LP), water-use efficiency (WUE), yield potential (YP), yield reduction (YR), yield index (YI), drought sensitivity index (DSI), stress tolerance index (STI), harmonic mean (HM), and geometric mean productivity (GMP). The genotypes with the least change in seed cotton yield under drought stress were Zeta 2, Delcerro, Nazilli 87, and DAK 66/3 which were also the most water-use efficient cultivars. The average genetic diversity of the panel was 0.38. The linkage disequilibrium decayed relatively rapidly at 20-30 cM (r 2 ≥ 0.5). We identified 30 different SSR markers associated with the traits. Fifteen and 23 SSR markers were linked to the traits under well-watered and water-stress conditions, respectively. To our knowledge, most of these quantitative yield and drought toleranceassociated loci were newly identified. The genetic diversity and association mapping results should facilitate the development of drought-tolerant cotton lines with high yield in molecular breeding programs.

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